1. Field of the Invention
The present invention relates to a technique of estimating a Doppler frequency for a mobile station in a mobile communication system.
2. Description of the Related Art
In a mobile communication system, a base station has a reception filter such as an Infinite-duration Impulse Response (IIR) filter. The base station uses the reception filter to extract only desired components from a signal received from a mobile station to demodulate the extracted components, which improves the demodulation characteristic at the base station. Moreover, the base station estimates a Doppler frequency of the received signal to control a filter coefficient of the reception filter based on the estimated Doppler frequency, thereby improving reception quality.
JP2003-198426 A (hereinafter, referred to as Document 1) proposes the following method to improve a throughput of the mobile communication system. According to this method, a base station estimates a Doppler frequency of a signal received from a mobile station. Then, the base station varies, in accordance with the estimated Doppler frequency, a multilevel number of a modulated signal to be transmitted to the mobile station.
As described above, in the mobile communication system, it becomes important to estimate the Doppler frequency for the mobile station with high accuracy.
For estimation of the Doppler frequency, JP 2003-508969 A (hereinafter, referred to as Document 2) describes the following technique. According to the technique described in Document 2, a base station obtains a channel estimate from a received signal for each slot. Then, the base station obtains the amount of change of the channel estimate as the phase variation amount of the channel, by using an inner product or a differential vector of the channel estimates. The base station estimates the Doppler frequency based on the phase variation amount.
FIG. 1 is a block diagram showing a configuration of a Doppler frequency detector for estimating a Doppler frequency based on a channel estimate.
The Doppler frequency detector shown in FIG. 1 consists a channel estimating unit 102, an inner product value calculating unit 103, and a Doppler frequency estimating unit 104. The channel estimating unit 102 estimates a channel based on a pilot signal received from a mobile station (not shown). After normalizing the channel estimate, the channel estimating unit 102 outputs the normalized channel estimate. The inner product value calculating unit 103 calculates the phase variation amount in the channel estimate in terms of time based on the output from the channel estimating unit 102. The Doppler frequency estimating unit 104 estimates a Doppler frequency based on the phase variation amount.
In the Doppler frequency detector shown in FIG. 1, the pilot signal received from the mobile station is supplied to the channel estimating unit 102. The pilot signal is supplied from a demodulating unit (not shown). The channel estimating unit 102 calculates a channel estimate h(m) through a processing expressed by Formula (1). The channel estimate h(m) is a complex number containing information on a phase and a reception intensity of the channel.h(m)=(1/K)·Σzc(m,k)D*(k) k=1,K  (1)where zc is a demodulated pilot signal, D* is a conjugate complex number of a known pilot signal, K is the number of pilot symbols in a slot of the pilot signal, k is a pilot symbol number and m is a slot number of a time slot of the pilot signal. According to Formula (1), the received pilot signal zc is multiplied by the conjugate complex number D* of the known pilot signal D. The numbers obtained by the multiplication are averaged in the slot. The channel estimates of the respective pilot symbols are averaged by the pilot symbol number K.
The channel estimating unit 102 normalizes the channel estimate h(m), and supplies the normalized channel estimate (h(m)/|h(m)|) to the inner product value calculating unit 103. In this case, |h(m)|is an absolute value of h(m). The inner product value calculating unit 103 calculates the phase variation amount θ by the following Formula (2).θ=cos31 1[Re{h′(m)·h′(m−n)*}]  (2)where
h′ (m)=h(m)/|h(m)|, and
h′ (m−n)=h(m−n)/|h(m−n)|.
In this case, Re{x} is a real part of the complex number x, and n is a calculation interval for obtaining the phase variation amount θ. The Formula (2) produces the same result as that of a processing in which the normalized complex numbers h′ (m) and h′ (m−n) are regarded as two-dimensional vectors and an angle formed between the two-dimensional vectors is obtained from an inner product value of the two-dimensional vectors. Therefore, hereinafter, “n” is also referred to as “inner product value calculation interval”.
To be specific, the inner product value calculating unit 103 calculates the phase variation amount θ from the channel estimate h(m) with the slot number m and the channel estimate h(m−n) with the slot number which is n slots antecedent to the slot number m. The phase variation amount θ can also be obtained by calculating an argument of h(m)/h(m−n).
The Doppler frequency estimating unit 104 estimates a Doppler frequency of the received signal based on the obtained phase variation amount θ.
FIG. 2 is a block diagram for explaining the outline of a technique described in JP 2001-24727 A (hereinafter, referred to as Document 3). According to this technique, a filter coefficient of an averaging filter for averaging the amounts of phase variation is controlled based on a Doppler frequency estimate.
A Doppler frequency detector shown in FIG. 2 includes the channel estimating unit 102, the inner product value calculating unit 103 and the Doppler frequency estimating unit 104 of FIG. 1. The Doppler frequency detector further includes an averaging filter 112 and a filter coefficient calculating unit 113. The averaging filter 112 averages the amounts of phase variation calculated by the inner product value calculating unit 103. The averaging filter 112 is formed of, for example, an Infinite-duration Impulse Response (IIR) filter. The filter coefficient calculating unit 113 controls filter coefficients of the averaging filter 112 based on the Doppler frequency estimate obtained by the Doppler frequency estimating unit 104.
The filter coefficient calculating unit 113 of FIG. 2 calculates the filter coefficient by using the Doppler frequency estimated by the Doppler frequency estimating unit 104 and a forgetting factor.
JP 2004-15819 A (hereinafter, referred to as Document 4) discloses a method of calculating an inner product value of pilot symbols and changing a filter coefficient for averaging the inner product value in accordance with the calculated inner product value to thereby determine a fading frequency based on the averaged inner product value. Document 4 also discloses that a base station calculates the inner products for a plurality of calculation intervals in parallel.
With the method of FIG. 1, however, the inner product calculation interval (n) for calculating the inner product is constant regardless of the Doppler frequency of the received signal. Therefore, when the Doppler frequency of the received signal is low, the resolution for detecting the phase variation amount becomes insufficient, which leads to lower the estimation accuracy of the Doppler frequency. On the other hand, when the Doppler frequency of the received signal is high, the calculation interval (n) becomes longer with respect to the estimated Doppler frequency, which also lowers the estimation accuracy of the Doppler frequency.
In the method described in Document 3, when the Doppler frequency is high, a time interval during which the averaging is performed becomes shorter. Therefore, a satisfactory effect cannot be obtained from the averaging.
Document 4 describes that, as stated above, the base station calculates a plurality of inner products for the plurality of calculation intervals, respectively. Therefore, the technique described in Document 4 requires a large amount of calculation. Moreover, in the technique described in Document 4, in a case where, for example, the fading frequency is high, the calculation of inner products at large calculation intervals serves for nothing. The wasteful calculation disadvantageously increases the device size of the base station.